1. Field of the Invention
The present invention relates to an inertial sensor and a fabrication method of an inertial sensor.
2. Description of the Related Art
In the related art, a vibrating gyroscope is known, which is processed using semiconductor process technique with materials such as silicon (Si). This type of gyroscope is one in which the inertial mass is vibrated in a predetermined direction to detect the magnitude of angular velocity by a displacement caused by the Coriolis force that occurs when the angular velocity enters. This angular velocity sensor can be applied to an input interface, and to the image stabilization of a video camera and a still camera, for example.
Generally, in such an angular velocity sensor, the vibration operation is hampered by the viscosity resistance of air surrounding the device, and the amplitude of the vibration becomes small, leading to a great decrease in the sensitivity of the sensor. It is necessary to increase drive voltage in order to obtain a desired amplitude of the vibration, causing problems of increased power consumption and an expensive voltage conversion circuit.
Because of these, a ceramic package, for example, is generally used to subject the sensor to vacuum encapsulation. However, in vacuum encapsulation with typical ceramic packages, when a degree of vacuum is increased too much, a serious problem interferes with the sensor characteristics due to influences such as a gas leakage into the package from outside, and gas produced inside the package. In addition, it is necessary to provide a robust airtight structure as the degree of vacuum is higher, which is a factor of a cost increase in the package.
For a technique which decreases the influence of the viscosity resistance with no pressure reduction (for example, see Patent Reference 1 (JP-A-2006-98168)), there is a technique which fills gas with an effective viscosity coefficient smaller than air. However, in the technique, only with regard to the atmospheric pressure, the Q value is about two times the air ratio at most, and generally, the effect is really small in the vibrating gyroscope which should have the Q value of a few hundreds to a few thousands.
In addition, the provision of vacuum encapsulation is performed to increase the Q value Qd on the drive side as well to reduce drive voltage and power consumption correspondingly. However, taking it into account that the Q value Qs on the detection side is increased at the same time, an over shoot occurs in the transient response at the time when the angular velocity is applied, which prolongs a time period necessary to reach the stable state, causing a problem at the same time that the response and a signal-to-noise ratio are reduced.
In addition, it is proposed that a sensor with non-linear characteristics is used to increase the drive amplitude (for example, see Patent Reference 2 (JP-A-2000-18951)). With the use of this method, the voltage on the drive side is reduced, and at the same time, control is allowed with no influence on the response on the detection side. However, in the technique, because the non-linear vibration is used, the amplitude might be greatly reduced due to a jump phenomenon, for example, when mechanical and electrical impact is applied, lacking stability.